This invention relates to computed tomography equipment and specifically to the correction of errors caused by variations in x-ray detector sensitivity along the z-axis.
Computed tomography (CT) systems, include an x-ray source collimated to form a fan beam, the fan beam extending generally along a fan beam plane and directed through an object to be imaged. After passing through the imaged object, the fan beam is received by an x-ray detector array extending along the fan beam plane. The x-ray source and detector array are rotated together on a gantry within an imaging plane, generally parallel to the fan beam plane, around the image object.
The axis of rotation of the gantry is designated as the z-axis of the Cartesian coordinate system and the fan beam plane and imaging plane is parallel to the x-y plane of the coordinate system.
The detector array is comprised of detector cells each of which measures the intensity of transmitted radiation along a ray from the x-ray source to that particular detector cell. At each gantry angle, a projection is acquired comprised of intensity signals from each of the detector cells. The gantry is then rotated to a new gantry angle and the process is repeated to collect a number of projections along a number of gantry angles to form a tomographic projection set.
Each tomographic projection set is stored in numerical form for later computer processing to "reconstruct" a cross sectional image according to algorithms known in the art. The reconstructed image may be displayed on a conventional CRT or may be converted to a film record by means of a computer driven camera.
Ideally, the fan beam plane will strike the center line of the detector array. In practice, however, the fan beam plane may be displaced along the z-axis from the center line because of two effects. The first effect is the thermal expansion of the x-ray tube's anode and its support. The surface temperature of the tube's anode may rise as high as 2000.degree. C. and the anode supporting structure may rise to 400.degree. C. or more. This heating and the resulting expansion of the tube's anode and its support causes a shifting of the focal spot of the tube which moves the point from which the x-rays emanate. The shifting of the focal spot causes a corresponding shift in the fan beam plane.
The second effect is the mechanical deflection of the gantry and anode support as the gantry rotates. This deforming stress results from the changing angle of gravitational acceleration and the changing magnitude of centripetal acceleration as a function of the rotational velocity of the gantry, acting both on the gantry and anode.
Displacement of the fan beam plane along the z-axis of the detector array is a problem because it causes variations in detector signals that are "exogenous" or unrelated to the internal structure of the imaged object. Generally each detector cell's sensitivity to x-rays will be a function of the z-axis position of the fan beam along the surface of that cell, that is, the detector cells exhibit a non-uniform "z-axis sensitivity". This z-axis sensitivity, combined with motion of the fan beam plane on the detectors, produces the undesired variations in the strength of the detector signal. Such exogenous variations in the detector signals produce undesirable ring like artifacts in the reconstructed image.
Compounding the problem of correcting for variations in z-axis sensitivity is the fact that the z-axis sensitivity generally differs among different detector cells in the detector array. This difference will be termed "intercell sensitivity variation".
Displacement of the fan beam plane and thus variations in the detector signals may be predicted and corrected. In U.S. Pat. No 4,991,189, issued Feb. 5, 1991, assigned to the same assignee as the present invention, and incorporated by reference, a control system using a movable collimator adjusts the z-axis position of the fan beam plane as deduced from a pair of special detector cells. The special detector cells provide information to a computer model of the system which in turn is used to control the collimator and to correct the placement of the fan beam plane. While such closed loop controls of the fan beam location reduce z-axis artifacts, they do not eliminate the problem.
Intercell sensitivity can be corrected using data from a calibration scan performed before a patient is in place. However, such corrections do not eliminate ring and band artifacts due to variations in detector sensitivity along the z-axis. Consider, for example, the z-axis sensitivity profiles of three different detector cells #1-3 in FIGS. 4(a)-4(c). Detector cell #1 represents a perfect sensitivity profile, while detector cells #2 and #3 represent actual sensitivity profiles with different characteristics. If these three detector cells are exposed to an x-ray flux which is uniform, the detector responses will differ because of the different z-axis sensitivities profiles, but these can be corrected using the calibration data.
Consider, however, the situation in which the x-ray flux is not uniform along the z-axis, but is instead variably attenuated by the patient being imaged. One such x-ray flux density profile is shown in FIG. 5(a), and the resulting response of these three detector cells after air calibration in FIG. 4 are shown in FIG. 5(b). On the other hand, consider a different x-ray flux density profile as shown in FIG. 6(a) and the resulting response of the same three detector cells after air calibration in FIG. 6(b). It is apparent that the corrections needed to level the detector responses is a function of the x-ray flux density profile, which in turn is a function of the attenuation characteristics of the object being imaged. In other words, the corrections needed depend on the object being imaged, and calibration data acquired with no object present will not suffice to eliminate ring and band artifacts caused by variations in the z-axis sensitivity of detector cells.